1. Field of the Invention
The present invention relates to a display device and a manufacturing method thereof. In particular, the present invention relates to a display device in which a thin film transistor is used for a pixel portion and a manufacturing method thereof.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (TFT) by using a semiconductor thin film (having a thickness of approximately several to several hundreds of nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors have been widely applied to electronic devices such as ICs and electro-optical devices and have been rapidly developed particularly as switching elements for image display devices.
A thin film transistor using an amorphous semiconductor film or a thin film transistor using a polycrystalline semiconductor film is mainly used as a switching element of an image display device. As a method for forming a polycrystalline semiconductor film, a technique is known in which a pulsed excimer laser beam is processed linearly by an optical system and an amorphous semiconductor film is irradiated with a linear laser beam while being scanned by the linear laser beam to be crystallized.
A thin film transistor using a polycrystalline semiconductor film has advantages that it has higher mobility than a thin film transistor using an amorphous semiconductor film by two or more digits, and that a pixel portion of a display device and a peripheral driver circuit thereof can be formed over the same substrate. However, because of crystallization of a semiconductor film, steps of a thin film transistor using a polycrystalline semiconductor become more complicated than steps of a thin film transistor using an amorphous semiconductor film. Therefore, there is a problem in that yield is reduced and cost is increased.
In addition, a thin film transistor using a microcrystalline semiconductor film is used as a switching element which has higher mobility than an amorphous semiconductor film and facilitates manufacturing steps (see Reference 1: Japanese Published Patent Application No. H04-242724, Reference 2: Japanese Published Patent Application No. 2005-49832, and Reference 3: U.S. Pat. No. 5,591,987).
As a method for manufacturing a thin film transistor using a microcrystalline semiconductor, a method is known in which an amorphous silicon film is formed over a gate insulating film, a metal film is formed over the amorphous silicon film, and the metal film is irradiated with diode laser to modify the amorphous silicon film into a microcrystalline silicon film (for example, see Reference 4: Toshiaki ARAI and others, SID '07 DIGEST, 2007, pp. 1370 to 1373). With this method, the metal film formed over the amorphous silicon film is a film for converting light energy of the diode laser into thermal energy and should be removed in a later step in order to complete a thin film transistor. That is, the method is a method in which an amorphous semiconductor film is heated only by conduction heating to form a microcrystalline semiconductor film.